1. Technical Field
The present disclosure relates to semiconductor devices that use nitride semiconductors such as gallium nitride.
2. Description of the Related Art
In recent years, active researches have been conducted on field effect transistors (FETs) that use nitride semiconductors, such as gallium nitride (GaN), as high-frequency high-power devices. Since GaN can form various mixed crystals with aluminum nitride (AIN) and indium nitride (InN), such a nitride semiconductor can form a heterojunction like arsenic semiconductors such as gallium arsenide (GaAs). However, even if the nitride semiconductor is not doped with impurities, highly concentrated carriers are generated by spontaneous polarization or piezo polarization at a heterojunction interface. Accordingly, an FET formed of the nitride semiconductor is likely to become a depletion-mode (normally-on) device, and it is difficult to obtain a characteristic of an enhancement-mode (normally-off) device (see, for example, Unexamined Japanese Patent Publication No. 2004-273486).
On the other hand, most devices currently used in a power electronics field are normally-off devices. Accordingly, an FET using a normally-off nitride semiconductor is strongly required in the power electronics field.
Examples of known structures of the normally-off FET using the nitride semiconductor include a structure with a simply decreased thickness or a simply decreased Al composition ratio of an aluminum gallium nitride (AlGaN) layer that serves as a barrier layer in an AlGaN/GaN heterojunction. The examples also include a structure in which a threshold voltage is shifted to a positive voltage by a dig of a gate section, and a structure in which a polarization electric field is not generated in a crystal growth direction of a nitride semiconductor by production of an FET on a (10-12) plane of a sapphire substrate.
A junction field effect transistor (JFET) that uses a p-type GaN layer for a gate is proposed (see, for example, Unexamined Japanese Patent Publication No. 2003-228320). The JFET uses, for a gate, a pn junction having built-in potential that is higher than built-in potential of a Schottky junction. This brings about an advantage that a rising voltage of the gate can be increased, and that a gate leakage current can be limited to a small level even if a positive gate voltage is applied.
Recently, efforts have been made to achieve high speed, low power consumption, space saving, and elimination of parasitic components in gate driver circuits, by production of power devices and gate driver circuits for switching power devices using these nitride semiconductors for integration into one chip as a power IC.
Unexamined Japanese Patent Publication No. 2012-151256 discloses a semiconductor apparatus that includes a first conductivity type field effect transistor and a second conductivity type field effect transistor on a semiconductor substrate.